We have prepared antisera in rabbits to the "contact sites A" glycoprotein (gp80) purified from Dictyostelium discoideum. IgG isolated these antisera reacts with a number of different proteins in D discoideum lysates, as analyzed by immune precipitation and by antibody staining of gel electropherograms transferred to nitrocellulose. blocking experiments indicate that this cross-reactivity reflects the presence of common antigeneic determinants on gp80 and other cellular proteins, rather than the presence of extraneous antibodies in the antisera. The spectrum of reactive proteins is different at different stages of development. In particular, gp80 itself is synthesized only for a restricted period during the cell aggregation phase. The protein persists throughout development and can be detected in spores. Anti-gp80 Fab fragments bind to the surface of developing D discoideum cells and specifically block their developmentally regulated adhesion. After absorption with vegetative cells, the IgG stains only gp80 and (to a lesser extent) one other band in lysates of aggregation-competent cells. The absorbed antibodies also can block adhesion. Several proteins that appear late in development also are stained by the absorbed IgG.